1. Field of the Invention
The present invention relates to an induction load driving system for driving an induction load of a stepping motor, etc..
2. Description of the Related Art
As a method of driving an induction load of a stepping motor, etc., for use with an optical disc device, a micro-step driving method is used. In the micro-step driving method, a ratio between amounts of currents flowing through a plurality of induction loads is changed finely, thereby controlling a rotational angle of a rotor with high accuracy.
In recent years, there has been a need for a smaller-sized optical disc device with reduction of a size of equipment incorporating the optical disc device, such as a notebook-size personal computer. With reduction of the size of the optical disc device, there has been a need for a smaller-sized stepping motor. As disclosed in Japanese Laid-Open Patent Application Publication No. 2007-87541, an optical disc device incorporates a small-sized stepping motor for moving an optical pickup, a smaller-sized stepping motor, etc., for moving a spherical aberration correction lens of the optical disc device using 3-wavelength lasers including a blue laser, etc.. Thus, these stepping motors are required to have a smaller size. Because of a limitation of a winding number, etc., of the induction load (coil) of the stepping motor, in these stepping motors, their inductances are lower than those of general stepping motors. Therefore, it becomes necessary to stably drive a current supplied to induction loads of low impedances.
Conventionally, there is known an induction load driving system used for the micro-step driving (see Japanese Laid-Open Patent Application Publication Nos. 2008-72876 and Hei. 11-215885). First of all, a configuration disclosed in Japanese Laid-Open Patent Application Publication No. 2008-72876 will be described. FIG. 7 is a schematic circuit diagram showing an exemplary conventional induction load driving system. As shown in FIG. 7, in the induction load driving system disclosed in Japanese Laid-Open Patent Application Publication No. 2008-72876, a current detection circuit 105 detects and feeds back a current flowing through a H-bridge circuit 102, 103 which uses a signal generated in an oscillator 108 as a carrier frequency in PWM (pulse width modulation) control and is capable of flowing a current bidirectionally through an induction load 101 (coil A,B) using four switching elements. The current detection circuit 105 detects a voltage applied to a detection resistor RS1, RS2 provided between the H-bridge circuit 102, 103 and a ground. A PWM signal generating circuit 104 compares a voltage value based on a current detected by the current detection circuit 105 to a voltage level of a maximum current setting signal, and generates a signal of a PWM driving waveform, to turn ON or OFF of the H-bridge circuit 102, 103, according to a result of the comparison.
Next, the configuration disclosed in Japanese Laid-Open Patent Application Publication No. Hei. 11-215885 will be described.
FIG. 8 is a schematic circuit diagram showing another exemplary conventional induction load driving system. In the induction load driving system disclosed in Japanese Laid-Open Patent Application Publication No. Hei. 11-215885 in a H-bridge circuit, a detection resistor 201 is connected in series with an induction load 202 to detect a current flowing through the induction load 202. A resistor R1, R2 converts the detected current into a voltage via an amplifier 203 as an average current flowing through the induction load 202, then an error amplifier 204 amplifies an error with a reference voltage Vref, and then a comparator 205, 206 compares the amplified voltage to a triangular wave which is a carrier frequency, thereby performing feedback control.